A communications network serves to transport information among a number of locations and typically comprises various physical sites or ‘nodes’, interconnected by information conduits, called “links.” Each link serves to carry information or data from one site to another site. Each site may contain equipment for combining, separating, transforming, conditioning, and/or routing data. These data may represent any combination of telephony, audio, video, or computer data in a variety of formats.
FIG. 1 shows an example communications network 100 comprising sites 101–105 connected by links 120–121. Links are generally implemented using electrical cables, satellites, radio or microwave signals, or optical connections and can stretch for tens or hundreds of miles between sites. Through these links, the communications network 100 carries data signals among the sites 101–105 to effectively interconnect data equipments 111–115, such as computers, remote terminals, file servers, etc. One or more links 120 and 121 that connect two sites are collectively referred to as a span 130. Sites 101–105 normally each comprise at least one cross-connect switch (either electrical or optical) and are in constant communication with a central network management system facility 140 which monitors the flow of traffic throughout the network.
Before the development of practical long-haul fiber links, a network, such as network 100, was commonly implemented in an all-electrical fashion using electrical cables or microwave paths as links in conjunction with switches and multiplexing equipment at the sites. A common high data rate signal to be switched and transported intact was the DS-3 signal, as standardized by the International Telecommunications Union(ITU), which carried data at around 45 megabits-per-second.
It is now preferable to use optical carrier signals to carry data along links from one site to another using optical fibers. Optical carrier signals are of such high frequency, around 1014 Hz, that they can be modulated at very high frequencies and can therefore carry data at an extremely high rate. For example, a standard SONET OC-192 modulated optical signal carries data at around 10 gigabits-per-second (10 Gbps).
FIG. 2 shows an example portion of a communications network wherein the links connecting sites are implemented as optical fiber links, yet the signals are switched in the electrical domain at each site. This may be referred to as an “optical/electrical” network. At each site the data-carrying signals are converted into the electrical domain to be routed through the digital cross-connect switches and perhaps processed in other ways. Collocated with the cross-connect switches at each site are so-called “lightwave terminal equipment” (LTE) which may comprise optical transmitters and receivers to couple data signals into and out of the optical fiber links.
In FIG. 2, a number of data signals to be transported are provided along data inputs 210 at a location called Site A. Digital cross-connect switch (DCS) 212 may combine and reformulate the data signals to yield a composite data signal along connection 222 to LTE 224. LTE 224 applies line-coding and may also add framing and automatic error correction information. LTE 224 may in some cases package asynchronous data signals into the payload envelope of a synchronous optical transport system. Once the signal has been prepared for transmission, LTE 224 then uses the line-coded data signal to modulate an optical carrier emitted from an optical transmitter 226, which usually comprises a current-modulated laser diode. The optical signal from transmitter 226 is coupled into optical fiber 228, which connects to distant Site B and may extend for tens or hundreds of miles. At various points along optical fiber 228, an optical amplifier, such as amplifier 230, or other means may employed to strengthen the signal and to compensate for degradation caused by imperfections in the optical path.
At Site B, the optical fiber is coupled to an optical receiver 232 which is a part of LTE 234. By techniques that are well known in the art, LTE 234 interprets the received optical signal and recreates at output 236 the same data content provided at connection 222, thus accomplishing transport of the data from one location to another.
At Site B, the received data along output 236 enters DCS 214 whereupon the received data stream may be partially demultiplexed, combined, and routed to be sent to other sites, or may be “dropped” to make the received signal available to destinations in the vicinity of Site B. Other optical links in FIG. 2 operate in a similar manner to the link just described.
Of further note, it is common for many optical links to be established between a given pair of sites. A set of links interconnecting two sites are collectively referred to as a “span.” Furthermore, it is common practice, particularly in telephony applications, to provide for corresponding pairs of directional links to be established between sites to accomplish bi-directional communications. A given LTE will often comprise numerous receivers and transmitters and may even couple multiple optical carriers, at different wavelengths, into and out of a single fiber.
In FIG. 2, the switching action of DCS 214 may accomplish redirection of individual data signals to either Site C or Site D. If a given data signal is introduced at Site A and is intended to be communicated to Site C, there are a variety of mechanisms to determine if the data signal is successfully reaching its destination. If the signal is disconnected or severely degraded due to a fiber cut or equipment malfunction, then electrical equipment, such as DCS 214, will not be able to synchronize with the signal (as is necessary to perform time slot interchange switching) and will declare a “loss of signal” or “loss of framing” alarm. The alarm indication will be reported whereupon a decision may be made to reroute the signal through an alternate link. It is fortunate that, in the electrical domain, the integrity of the signal is inherently checked at each point where the signal is received or switched. This allows for pinpointing the location of a failure and for deciding effective actions to circumvent a failure in the network.
For example, if LTE 234 or DCS 214 cannot detect or achieve synchronization with the signal from Site A, then an alarm is generated and reported to a network management system, such as system 140 as was shown in FIG. 1. Based upon other alarms from LTE 234, or even LTE 224, the network management system may determine that a failure has occurred, along fiber 228 for example, and may direct DCS 212 and DCS 214 to utilize optical fiber 240 as an alternate link.
As another example, assume that LTE 234 and DCS 214 indicate successful receipt of the signal incident along fiber 228, yet LTE 244 or DCS 216 indicate loss of the signal. These conditions are reported by the various elements to network management system 140 and correlated to determine that the failure is along fiber 242 or at LTE 246.
The hybrid optical/electrical approach depicted in FIG. 2 is presently in widespread use in the industry and offers substantial advantages over the older all-electrical systems. However, it is further desirable, for many practical reasons, to route modulated optical signals through a network entirely in optical form, that is, without having to convert an optical signal into an electrical equivalent until it reaches its destination.
Conversion of an optical signal into the electrical domain introduces many limitations. At each point where a modulated optical signal is received and converted into an electrical equivalent, the specific data rate and format, and in some cases the specific carrier wavelength, must be established so that the receiver is capable of accommodating the incoming signal. Aside from the hardware costs involved in receiving and re-transmitting an optical signal, the conversion to an electrical signal restricts the type of optical signals that may be carried through the network. When an upgrade to a higher data rate or different modulation format is desired, the electrical domain equipment handling signals must be changed. Furthermore, the conversion to an electrical signal limits the ability to handle a variety of signal bandwidths and formats which may be carried simultaneously within the same optical network. Restoration options are thus limited in the event of a sudden failure in the network. This was not such an issue in the older electrical networks that carried DS3 signals almost exclusively throughout.
Because of these limitations, manufacturers and network owners are striving to deploy completely transparent all-optical networks using optical cross-connect switches. These types of switches simply couple one optical path to another without having to receive or transduce the optical signal into an electrical signal. Regardless of what optical signals or modulation formats are propagated down the fiber, the optical carriers are routed by the optical cross-connect switches. Upgrades to higher data rates or formats can occur without any changes to the core network switches. Mixtures of data rates and formats are readily accommodated in a transparent all-optical network. It is desirable to create a transparent “core network” of optical cross-connect switches to carry and switch extremely large traffic channels.
It should be noted that some varieties of optical cross-connect switches are entirely transparent whereas others perform routing depending upon carrier wavelengths. However, both varieties are advantageous for being independent of the data modulation employed upon each optical carrier.
An example of a portion of an all-optical network is shown in FIG. 3 and maybe compared to the optical/electrical system of FIG. 2. Data signals presented for transmission at data inputs 310 are routed and combined into aggregate high-data rate signals within DCS 312 and electrically coupled to LTE 316 along connection 314. LTE 316 comprises optical transmitter 318 that emits an optical signal modulated with the data supplied by DCS 312. The modulated optical signal from transmitter 318 propagates through optical fiber 320 to eventually reach Site B.
At Site B, the optical signal is coupled into an input port 338 of an optical cross-connect switch (OCCS) 350 to be routed to one of many possible output ports. The switching action of OCCS 350 determines how each signal at an input port is redirected to a particular output port. And, because the output ports of a given OCCS may lead to many different remote sites, the switching of OCCS 350 accomplishes routing of optical signals to different physical destinations. In the present example, OCCS 350 may establish a light path between input port 338 and output port 340, effectively passing the signal from fiber 320 into fiber 328. This causes the optical signal from transmitter 318 to be received at receiver 330 in LTE 332, meaning that the data from input 310 and DCS 312 is available through DCS 334 and at output 336.
At Site B in FIG. 3, optical amplifier 322 is inserted in the optical path to boost the signal before entering OCCS 350. Some types of OCCS use a lossy switching matrix and it is advisable to pre-amplify weak signals before entering the switch. Optical amplifier 326 represents the common practice of amplifying optical signals after leaving an OCCS and upon reentering a fiber link. This compensates for losses experienced through the switch and provides a power boost to launch the optical signal through a long fiber link to the next site.
While the all-optical approach shown in FIG. 3 offers many worthwhile advantages, it introduces some new challenges. As described earlier, the traditional electrical networks and the more recent optical-electrical networks always received and interpreted at least portions (i.e. framing and parity information) of the data signal. Detection of the integrity of each data signal was inherently necessary at each point where the data signal was received, switched, or regenerated.
In contrast, in a transparent all-optical network approach, these aspects of the data signal are not routinely sampled. An optical cross-connect switch, such as OCCS 350, operates “blindly” without regard for the presence or absence of optical signals at its input and output ports. A malfunction in OCCS 350, or a mistaken instruction that controls OCCS 350, could cause an optical signal to be dead-ended or to be incorrectly routed to another site. In a network of optical cross-connect switches, the routing of a given signal is accomplished by issuing commands to several cross-connect switches, but there is generally no mechanism for verifying the proper routing of the optical signal except at its final destination.
Typically, a centralized or moderately distributed provisioning function coordinates the action of the cross-connect switches to accomplish routing of optical signals. The provisioning function usually maintains a database describing how the switches are interconnected in the network and relies upon the stored data to decide what switching commands to issue to the switches. Optical cross-connect switches are presumed to work properly, just like their electronic counterparts, and the database is assumed to accurately represent the interconnections in the network. But if a switch fails to connect ports in response to a command or the database inaccurately shows a link where none exists, then an optical signal may not reach its intended destination. Furthermore, there will be no indication of where along the path the optical signal has been misrouted. This problem may be exacerbated when restoration switching actions occur in the network that temporarily alter the connection topology.
What is required is a means for verifying, in a network of optical-domain switches, that optical data signals have been correctly switched and routed as intended and that the optical switching mechanisms are working properly. Furthermore, a means is desired for determining the location of a malfunctioning element so that traffic may be routed around it and repairs can be readily initiated. It is also desirable that any malfunctions within the switching mechanism of an optical switch be detected and noted locally so that the switch may declare a localized alarm or may alter its internal routing logic to circumvent the failure.